The present invention relates to the continuous casting of steel. It relates more particularly to preventing the casting nozzle from being blocked when casting a slab or strip made of killed steel, especially low-carbon steel or ultralow-carbon steel (called ULC steel or IFS).
It is known that the continuous casting of semi-finished products of wide cross section (slab, thin slab, strip, etc.) conventionally requires the use of a submerged nozzle to feed the casting mold with molten metal from the tundish placed above it.
It is also known that these nozzles are subject to fouling resulting, in the relatively long term, in them being completely blocked and, consequently, resulting in the casting run in progress being immediately stopped.
It will be recalled that fouling is a phenomenon involving the gradual narrowing, from the periphery toward the centre, of the pipe that the nozzle offers the liquid metal in order for it to pass into the mold. The origin of this phenomenon is the deposition of solid particles on the inner wall of the nozzle, these particles being non-metallic inclusions from the deoxidation of the liquid metal. These inclusions are already present within the molten metal following the metallurgical treatments undergone beforehand by the latter, or they form during actual flow through the nozzle if the latter is not sufficiently impervious to the oxygen from the ambient atmosphere. The number and the volume of these non-metallic inclusions vary with the steel grades cast, as does the extent to which they solidify at the temperature of the molten metal.
In this regard, it is known that serious castability difficulties may arise, particularly in the case of the casting of low-carbon steel or ultralow-carbon steel (of the IFS type, for example), and therefore in highly killed steel.
Conventionally, steels of this type are killed in the refining ladle by the addition of aluminum, this being a deoxidizing agent widely used in iron and steel manufacture. The deoxidation reaction produces aluminates which predominantly settle on the surface of the molten metal, firstly in the ladle and then in the tundish. However, some of these non-metallic inclusions inevitably remain suspended within the mass of liquid metal at the moment of casting. In particular, it is these particles which, during their transit through the nozzle, become attached to the wall of the pipe and, via an accretion phenomenon over time, end up by blocking the passage.
It is known to prevent these blockages by making a stream of inert flushing gas (especially argon) flow through the nozzle. The mechanism, or more probably the mechanisms, via which such a gas flush counteracts the fouling has not yet been fully elucidated, but the result is generally rather satisfactory if the bubbling is installed right from the onset of the casting run. Otherwise, clumps of inclusions may become detached and contaminate the metal in a dramatic fashion, making this practice a remedy worse than the disease.
However, the method, even correctly applied, is not without undesirable side effects. Defects of the xe2x80x9cblisterxe2x80x9d type may appear on strip during subsequent rolling, which are known to result in the phenomenon of gas bubbles being trapped within the in-mold solidified metal.
It is also known to prevent nozzle blockages by means of preventative measures, the primary benefit of this being to be able to dispense with the xe2x80x9cargon bubblingxe2x80x9d. One of these measures consists in adding a flux, such as Ca (for example in the form of Sixe2x80x94Ca or Caxe2x80x94Fe), to the molten metal before casting, and therefore in the tundish, or preferably already in the refining ladle, which flux will complex with the deoxidation aluminates to form more meltable inclusions, these therefore remaining in principle in the liquid state at the casting temperature. A preventative treatment of this type, by addition of calcium, is described for example in the document EP-A-0 512 118, the overall teaching of which will be considered as being incorporated into the present specification by reference.
However, such chemical treatment of the blocking does not always give the expected results. This is because it sometimes happens that the inclusions formed, even in the presence of calcium, are already in the solid state in the tundish, this being so even in the case of casting with significant overheating of the metal.
The object of the invention is specifically to achieve better fluidity of the oxidation inclusions that have formed by the calcium treatment of the molten metal before casting.
For this purpose, the subject of the invention is an in-ladle metallurgical treatment of a steel having to be continuously cast, in which calcium is added to a molten ultralow- or low-carbon steel which has been killed (or is in the process of being killed) with aluminum in order to achieve a given oxygen content, so as to form deoxidation inclusions having a melting point below the temperature at which the steel is cast in the mold, wherein the molten metal is maintained, within the treatment sequence going from the ladle to the casting mold, with a dissolved magnesium content close to at least 2 ppm, without exceeding the content, which depends on the oxygen content of the molten metal, above which solid magnesium-based spinels may form.
As will have been understood, at the basis of the invention is the discovery of the beneficial action of magnesium, in small amounts, in keeping the deoxidation inclusions in the liquid phase, whether these be present after killing or formed during casting in the presence of calcium. This is because it has been shown that the presence of magnesium in small amounts (namely at least about 2 ppm of Mg, and possibly up to 8-10 ppm for the oxygen contents usually encountered in aluminum-killed low-carbon or ultralow-carbon steels) within a calcium-treated molten metal has an influence on the physical nature of the inclusion population in the cast steel: the element magnesium considerably broadens the range of existence of liquid calcium aluminates at the casting temperature of the steel (approximately 1520-1570xc2x0 C.). It should also be emphasized that such broadening is very sensitive to the presence of magnesium even in very small amounts, a small variation from a very low Mg content (a variation of less than 1 ppm) possibly resulting, as will be seen, in a consequent broadening of the meltability range.